"Divorce can be a trying, draining, and emotionally devastating experience for all involved," Pratt, 27, tells PEOPLE. "Despite these hardships, most individuals manage to conduct themselves with dignity and maturity. I did not – I failed spectacularly."

Addressing his actions toward Montag, 23, he says, "I horribly embarrassed the one person that meant the world to me. For my outrageous and infantile behavior, I offer Heidi my most sincere apologies."

He didn't specify what actions, but Pratt has been threatening to release a sex tape involving Montag, launched rants against her on Twitter and has been relentlessly pursuing her in Costa Rica.

"Over the years, her love, support, and friendship were always given, but never properly cherished. I let her down," he says. "She never deserved the pain and anguish I put her through, and although I don't expect it, I hope in time she'll be able to forgive me."

He adds, "Even though our relationship has changed, I will always count her among my best friends."